Disk files are information storage devices which utilize a rotatable disk with concentric data tracks containing the information, a head for reading or writing data onto the various tracks, and an actuator connected by a support arm assembly to the head for moving the head to the desired track and maintaining it over the track centerline during read or write operations. In high capacity disk files there are typically a plurality of disks stacked on a hub which is attached to a rotatable shaft. The shaft or spindle is driven by the disk drive motor and is supported at its ends by bearing assemblies which are mounted to the disk file housing. The data on the disks are accessed by heads which are part of head-arm assemblies connected to the actuator. In such disk files, multiple actuators may be used to position the heads to their respective disks in the stack. The movement of the head to a desired track is referred to as track accessing or "seeking", while the maintaining of the head over the centerline of the desired track during a read or write operation is referred to as track "following".
The actuator is typically a "voice coil motor" (VCM) which comprises a coil movable through the magnetic field of a permanent magnetic stator. The application of current to the VCM causes the coil, and thus the attached head, to move radially. The acceleration of the coil is proportional to the applied current, so that ideally there is no current to the coil if the head is perfectly stationary over a desired track.
In disk files which have a relatively high density of data tracks on the disk, it is necessary to incorporate a servo control system to maintain the head precisely over the centerline of the desired track during read or write operations. This is accomplished by utilizing prerecorded servo information either on a dedicated servo disk or on sectors angularly spaced and interspersed among the data on a data disk. The servo information sensed by the read/write head (or the dedicated servo head if a dedicated servo disk is used) is demodulated to generate a position error signal (PES) which is an indication of the position error of the head away from the nearest track centerline.
In a disk file digital servo control system, a microprocessor utilizes a control signal algorithm to calculate a digital control signal based upon the digital values of certain state variables such as PES, VCM current and head velocity. The digital control signal is converted to an analog signal and amplified to provide input current to the VCM. Such a digital servo control system is described in U.S. Pat. No. 4,412,161, wherein the digital control signal is calculated recursively from prior control signals and prior values of the PES.
Assignee's U.S. Pat. No. 4,679,103 describes a digital servo control system which, as part of the computation of the control signal to the actuator, makes use of a state estimator algorithm to estimate the position and velocity of the head. In this type of system, a microprocessor receives, at discrete sample times, digital values corresponding to the PES and the actuator input current, and computes, through the use of the state estimator algorithm, a digital control signal. The digital control signal is then converted to an analog signal and amplified to provide a new actuator input current. The method of estimating the state of the physical plant to be controlled in such a digital servo control system requires the use of estimator constants, the derivation of which is described in Digital Control of Dynamic Systems, Franklin and Powell, Addison-Wesley Publishing Co. (1983), chapter 6, pages 131-139. Assignee's copending application, Ser. No. 249,619, now U.S. Pat. No. 4,914,644, describes a digital servo control system which eliminates the need to measure actuator input current, so that the only variable input to the control system is the digital PES.
There are several causes for the head being off the track centerline during track following and which thus contribute to the PES. Certain position error components are synchronous with disk rotation and are thus repeatable. For example, if the disk is not precisely centered over the axis of rotation of the spindle, the circular tracks will have an eccentric shape relative to the axis of rotation. This will cause a repeatable disk runout error at the same frequency as the rotation of the disk. Assignee's U.S. Pat. No. 4,616,276 describes a servo control system which rapidly removes repeatable error from the PES. Other position error components are not synchronous with disk rotation and occur at a frequency significantly higher than the disk rotation frequency. For example, instability in the drive motor bearing assemblies will contribute a nonrepeatable error component to the PES. Such nonrepeatable runout (NRRO) does have a dominant sinusoidal behavior, although the amplitude, frequency and phase (relative to the disk index mark indicating the beginning of a track) of the runout does not repeat from one disk rotation to the next. German patent application DE 36346 describes a disk file having a special head which detects nonrepeatable wobble of the spindle from signals recorded on an auxiliary track. The signals from the auxiliary track are used to control the position of the data read/write heads during reading and writing of data on the data tracks.